In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh, papermaking fabric, or wire and water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is, in essence, a water removal, system. A typical forming section of a papermaking machine includes an endless traveling papermaking fabric or wire, which travels over a series of water removal elements such as table rolls, foils, vacuum foils, and suction boxes. The stock is carried on the top surface of the papermaking fabric and is de-watered as the stock travels over the successive de-watering elements to form a sheet of paper. Finally, the wet sheet is transferred to the press section of the papermaking machine where enough water is removed to form a sheet of paper.
In tissue making, the endless forming fabric is conveyed onto a turning Yankee dryer cylinder which is heated internally by steam to dry the web. A hood over the Yankee is supplied with the heated gases, often including combustion exhaust, which dry the web and remove the moisture evaporated therefrom. The dried web is detached from the Yankee cylinder by a doctor blade that extends across the width of the machine. This operation simultaneously crepes or crumples the web in the machine direction, so that micro-folds are formed, whose folding axis is substantially perpendicular to the machine axis. The creping does not impose single folds extending across the whole web, rather an apparently random pattern of interlaced and highly elongated branching folds each extending for distances of millimeters to centimeters across the machine, and with a distribution of folding pitches typically of some hundreds of microns. On the hood side, this folding gives the tissue surface a furrowed appearance of rolling relief, while on the cylinder side the folding tends to resemble a smoother surface of low relief crossed by narrow seams. The creped tissue is then conveyed with minimal contact of other apparatus to a reeling device. The mechanical and aerodynamic forces applied in conveying usually stretch the tissue by partial unfolding of the crepe, such that the crepe pitch is typically from 300-900 microns at the reeler. As is apparent, the quality of the tissue is determined in part by the creping operation on the tissue machine.
FIG. 1 is an image of the hood surface of a commercially available one-ply tissue which shows a network of ridges that are separated by furrows. One measure of the crepe pitch or crepe scale is to determine by the number of ridges or furrows per unit length along a linear path, e.g., line H, which is shown as being parallel with the machine direction. Most ridges, e.g., A, F and G, and furrows are essentially or nearly straight. Branching at various junctures, e.g., B, C, and E, occurs so that groups of ridges or furrows can be connected. The lengths, widths, and angles of individual ridges or furrows, or sections of connected ridges or furrows are distributed over some range of values.
The crepe pattern and especially the pitch of crepe folds or seams is an important product quality indicator for tissue. Crepe structure is related to properties of bulk, softness, and absorbency in tissue, which greatly influence its utility for a purpose, and customer satisfaction with tissue products. The crepe pattern is also an important process quality indicator for tissue-making machines. Deviations in crepe pattern either locally or across the whole sheet can indicate a range of process problems, such as excessive wear on the doctor blade, or improper flow of additives to the Yankee cylinder.
Timely information on crepe structure or changes in crepe structure allows the process to be promptly adjusted so that an acceptable product is made at all times. Such adjustments can include changing the angle or pressure of the doctor blade, or changing the amount and relative proportions or spatial distribution of water, glue, and other agents which are continuously applied to the Yankee cylinder surface. Moreover, knowledge of the crepe structure allows optimal scheduling of doctor blade changes, which is a costly operation requiring an interruption to production.
U.S. Pat. No. 5,654,799 to Chase et al. discloses a device for making very high speed measurements of the smoothness of a moving sheet such as paper; the patent purports that the apparatus, which employs an on-line laser triangulation position sensing system, can be used to infer the crepe wavelengths of tissue. However, in this mode of operation, the device requires that the tissue path be confined to a very narrow plane by a sheet stabilizer in the immediate vicinity where detection is made, which is problematic. Strict confinement of the web path would require either significant web tension or other direct mechanical forces, or substantial aerodynamic elements such as forced vortices or blowboxes which induce indirect tension. In any of these scenarios, the force applied can easily lead to decreping or even to web disruption, since most grades of tissue are not strong, and the crepe folds are only weakly bonded. The forces involved also increase nonlinearly with the speed of the web, thereby exacerbating the difficulty at the high speeds of modern tissue machines. Moreover, even if the mechanical difficulties could be overcome, measurement of crepe from surface roughness would be useful only on the hood side of the web, which is the surface of the tissue away from the Yankee cylinder. The reason is that the creping process is asymmetric which forms curved surface folds on the hood side and seams in an otherwise smoother surface on the cylinder side.
U.S. Patent Application 2005/0004956 to Pourdeyhimi discloses a method of evaluating surface texture in which surface features and defects of samples are said to be deduced by analyzing digitized images of illuminated surface. Specifically, the technique is directed to detecting selected surface and physical optical properties such as fiber orientation distribution and basis weight non-uniformity (blotchiness) of carpets. While the application states parenthetically that paper structures can also be evaluated, the reference provides no details to accomplish this. Moreover, that application teaches exclusively the use of two-dimensional Fourier analysis for its quantification of orientation distributions, whereas as will be disclosed below, with the present invention, analysis of crepe folding orientations is performed by a non-spectral method.
At present, there is no known method to measure the crepe structure until a sample can be taken from a finished reel to the laboratory for analysis. This leads to delays between the occurrence of a product quality disturbance or process defect and its detection. Thus remedial actions are delayed and considerable quantities of unacceptable tissue may be produced. The industry is in need of techniques for on-line crepe structure measurements with the goal of maintaining output quality and minimizing the quantity of product that must be rejected due to disturbances in the manufacturing process.